This invention relates to weighing scales that utilize load cell assemblies. Particularly, this invention relates to improved load cell assemblies used in electronic wheel scales.
A co-pending U.S. patent application entitled, "Wheel Scale Assembly" by Kroll et al has also been filed. That application further describes and claims wheel scale assemblies with which the load cell assemblies of this invention are used.
Wheel scales or platform scales are commonly used to measure axle weight loads of vehicles, such as trucks. And, although some platform scale devices are designed to accomodate the multiple wheels of dual tandem rear axle assemblies of commercial trucks, most wheel scale devices have a single platform to measure the load from single or dual wheels.
In use, a pair of wheel scales are spaced apart and the vehicle is driven onto the scale platforms one axle at a time. Each axle weight load is recorded and the summation of axle weights yields the gross weight of the vehicle.
Wheel scales utilizing load cells for weighing axle loads of motor vehicles are known in the weighing art. Additionally, wheel scales which utilize strain guage sensors fixed to deflectable load cells are known. However, the specific configuration of the load cells utilized in prior art scales, the placement of the load cells in the housing structures of these scales, and the cooperation of the active and inactive elements of these prior art wheel scale devices often yield complex and bulky wheel scales which are inaccurate and unrepeatable under many conditions of use.
For example, prior art wheel scale devices have been proposed and manufactured to use various types of load cell configurations for the purpose of yielding a lightweight, low profile and accurate portable wheel scale. Additionally, various wheel scale structures have been proposed and manufactured wherein these load cell configurations have been utilized with varying cooperative elements. Unfortunately, these prior art wheel scale structures have invariably resulted in devices that are not only difficult to transport and utilize and which are easily susceptible to damage, but which have required precise load placements onto the scale platforms to achieve any scale accuracy and repeatability.
Although load cell assemblies are usable in a variety of vehicle scale devices, there are specific requirements and difficulties associated with their use in portable wheel scales used by law enforcement officers, for example. Such portable wheel scales must be unitary, lightweight and rugged assemblies able to withstand the lateral forces resulting from the braking and skidding of truck tires.
Additionally, such portable wheel scales are preferably usable on a variety of road surfaces or terrains, should have low or thin profiles to prevent weighing inaccuracies caused by load shifts and are preferably accurate and reliable irrespective of the precise wheel placement of a vehicle onto the platform structures. The load cell assemblies of this invention in conjunction and cooperation with the scale housing structure and load cell retaining structure provide such a reliable and accurate portable wheel scale.
Many types of load cell assemblies have been used or proposed in conjunction with weighing scales as well as portable wheel scales. And, although ideally a freely floating load cell assembly is preferred for accuracy in weighing, it is also necessary to restrain the movement of the load cell itself for proper usage. This is particularly important in portable scales. Consequently, a variety of restraining mechanisms have been proposed to maintain the proper placement of the load cells within the confines of their respective wheel scale body structures. However, the use of these load cell fastening or restraining structures often result in unreliable load cell behavior and, therefore, in undesirable variations in the output of the strain gauges used in conjunction with them. And, although efforts have been made to correct or compensate for these load cell fastening structures, such compensation has been found difficult to maintain over periods of continued scale use due to physical changes in the fastening stuctures themselves. The load cell retaining structure of this invention provides a means for reducing the effects of this longstanding problem in the portable scale art.
Despite the need for a truly portable, low profile, sturdy and accurate wheel scale device which overcomes the problems associated with these prior art weighing scales, none in so far as is known, has been proposed or developed. Accordingly, it is the object of this invention to provide a lightweight, fully electronic, self contained, high capacity wheel load scale which has a low profile for easy use and which utilizes a load cell assembly and cooperating load cell retaining structure and housing assembly that provides for the accurate and repeatable weighing of axle loads, for example, under a wide range of environmental conditions.
It is particularly the object of this invention to provide a load cell assembly for use in weighing scales. The load cell assembly provided is a metallic load cell beam which is rectangular in structure. The load cell beam is designed to cooperate with the platform of a weighing scale which is directly in communication with it. The uniform, distributed load exerted by the platform to the load cell is supported by the bearing assemblies located outside of the axially disposed apertures having stress isolation webs therebetween. These stress isolation areas are subjected to a maximum shear because of their locations with respect to the distributed load.
The load cell assemblies of this invention are generally continuous, horizontal beams that are unrestrained. The load cell beams also have localized I-beam constructions for structural integrity and for shear stress measurement purposes at the placement of the strain gauges. The opposing pairs of angularly and transversely mounted strain gauges are mounted to the parallel rear walls in the axially disposed apertures in the beam body and which importantly are located at the outside edges of the distributed load area on the load cell due to the weight transfer section of the weighing platform.